A Sue tracker is a device used to monitor the more Suvian agents, be they ex-Sues or agents dangerously close to becoming Sues. Trackers seem to be able to be programmed to do a number of things, from simply alerting the agent that they are close to using Sue powers, to acting as a power dampener. It can be assumed that the trackers are also used to monitor an agent's location.
Trackers tend to take the form of a metal bracelet or anklet, and typically can't be unlocked without authorization.
Agents Diocletian, the Aviator, Shamrock Alsike, and Jennifer Robinson are known to have been fitted with Sue trackers either permanently or temporarily.